Most marine craft presently are propelled and controlled by single fixed propulsor devices such as screws and control surfaces. This has limited the placement of shipboard internal propulsor machinery that drives the propeller and has required additional drag producing motion control appendages such as rudders and diving planes. A small number of other marine craft use alternate mechanical propulsors that use propulsion units that change a propeller's cyclic and collective blade pitch that in turn generates thrust vectors in up to six degrees of freedom. Such propulsors use a swash plate for the pitch control mechanism. Applications of this include ship secondary propulsors that use a Voith-Schneider propeller and underwater submersibles. Particular means of mechanically varying the propeller blade pitch have included the use of a swash plate ring that floats on rubber or steel spring mounts to permit freedom of movement when the actuators push and pull the ring along the axis of rotation and when the ring is angled to the axis of rotation. This is shown in U.S. Pat. No. 3,101,066 of Haselton titled "Submarine Hydrodynamic Control System". Limitations and disadvantages of this system include: 1) an admission in U.S. Pat. No. 3,450,083 Background section describing the prior propulsion system in U.S. Pat. No. 3,101,066 as "lacking durability" and exhibiting "much lost motion", 2) this system is limited to use in a tandem propeller type propulsion configuration that precludes its use on other marine vessels such as a surface vessel; 3) the ring ac propulsion motor as shown does not incorporate both the swash plate control mechanism and actuator means in the central bore section of the motor that makes for an integral propulsion unit; and 4) the floating swash plate ring requires a large open bore structure for its placement which induces lost motion.
Haselton's U.S. Pat. No. 3,450,083 titled "Submarine Hydrodynamic Control System", discloses an improved way of supporting the swash plate ring which controls the cyclic and collective pitch of a many bladed propeller with an internal ring gear drive mechanism. Limitations and disadvantages of this system include: 1) the drive motors use of rotating shaft seals that may leak; 2) the need for a conventional in-line motor for driving the bull gear drive unit, thus requiring more internal marine vessel space; and 3) this system is limited to use in a tandem propeller type propulsion configuration that precludes its use on other maine vessels such as a surface vessel.
Other propulsion units developed and is closed include Kawasaki Heavy Industries' Varivec propeller unit which consists of a swash plate that is actuated by servo-actuators which alters each propeller blade's combined cyclic and collective pitch that generate thrust in any direction. Limitations and disadvantages of this unit include: 1) the need for a rotating shaft seal on the electric motor; and 2) and in-line electric motor/speed reducer unit that requires more internal vessel space, thus limiting the flexibility for drive unit placement on a marine vessel.